Linear perspective radar systems for approach and landing of aircraft have been disclosed in my previous U.S. Pat. No. 3,778,821, issued Dec. 11, 1973 of which the aforementioned parent application Ser. No. 385,534 is a continuation-in-part.
Essentially, the basic concept of the perspective radar system as disclosed in the above-mentioned U.S. Pat. No. 3,778,821 is the display on a radar scope of a view of targets in front of the aircraft such as received from the terrain or an airport complex in perspective which coincides exactly on a one-to-one basis with a visual view of such terrain or airport complex on a clear day.
While the conventional perspective radar view as well as the actual visual view aids a pilot greatly, during taxi or roll-out of an aircraft the perspective radar as well as the visual perspective view has limited use. It is a common occurrence for pilots on a clear day to be directed where to taxi by the control tower when the airport is unfamiliar to the pilot because the pilot is so low to the ground, particularly in smaller aircraft, that the various taxi-ways and runways are difficult to visually detect. The same situation obtains if the perspective radar is used since all of the targets are compressed close to the horizon and cannot readily be distinguished. Similar conditions exist in the case of boats or ships where a perspective radar might be used but wherein the height of the antenna is at a relatively low altitude. With the use of a perspective radar such as described in my referred to prior U.S. patent, other target ships or boats would appear compressed together close to the horizon as compared to their appearance as targets if the radar and observer were positioned at a substantially higher altitude.
A further limitation of the conventional perspective radar is the ability to detect close in targets. As an aircraft or boat approaches a runway or a mooring, the nearer video signals will disappear from the display simply because the time for the radar pulse to reach the target and return is so short as not to be detectable at the starting of the vertical sweep of the display tube.